Moment excitation and the measurement of moment mobilities

Frequency response functions (FRF), such as mobilities, are widely used in the analysis of vibration and structure-borne sound and it is important that this FRF data can be measured accurately for all important degrees of freedom. In some cases three translational and three rotational components of both excitation and response may be of importance; i.e. three forces and moments, and three velocities and angular velocities. Of these, the measurement of angular velocity due to moment excitation is one of the most challenging. This paper describes a known approach, sometimes referred to as the central difference method, which can be used for this purpose. The central difference method is thought to be one of the most practicable methods for measuring moment mobilities because it avoids the need for a moment exciter; instead finite differences are used to approximate the moment mobility which is a spatial derivative of the more easily measured velocity to force mobility ratio. There does however remain some doubt regarding the accuracy of the central difference method because of the finite difference approximation made and the method's possible susceptibility to random and bias errors. To better understand the finite difference error, an error analysis using a Taylor series expansion and simulated experiments for plate and beam structures are provided. It is then argued that random and bias errors associated with the measurement chain should now, with modern instrumentation, be less of a problem. An experimental validation of the method using two approaches is used to test this hypothesis. It is concluded that the central difference method provides a good balance between measurement effort and data quality making it widely applicable.     

Error due to two-force excitation in moment mobility measurement

In direct moment mobility measurement, one difficulty is the moment excitation. Two-force configuration excitation is widely used to generate moment in practice. Previous error analysis mainly focused on the initial load of exciters, the matching of the two forces, and the system resonance. In this paper, the distance requirement of two-force configuration in moment excitation is investigated. Based on the numerical calculation, other factors that affect the error in moment mobility measurement are discussed. The influence of the deduced distance requirement on moment mobility measurement is also explained.     

The static quadrupole moment of the first excited state of 198Hg

The first direct evidence of oblate nuclear deformation in the even-A mercury isotopes has been obtained by measurement of the static quadrupole moment of the first excited state of ¹⁹⁸Hg using the reorientation effect in Coulomb excitation.     

LHCD系统微波激励源性能优化设计

使用数字锁相环技术实现一定带宽内高精度可调节的频率输出,解决了速调管长期工作后响应频率发生偏移,模拟锁相环无法工作在系统最佳频率的问题;在移相器后增加功率幅度控制电路,保证了线性放大器的输出功率不会受到相位调节的影响;设计了一种基于数模转换(D/A)技术与微波电调技术相结合的波形控制单元,解决了激励源输出波形不可控的问题。通过以上三方面的改进,完成了HL-2M装置LHCD系统微波激励源的性能优化设计。     

Higher-order quadrature-based moment methods for kinetic equations

Kinetic equations containing terms for spatial transport, body forces, and particle–particle collisions occur in many applications (e.g., rarefied gases, dilute granular gases, fluid-particle flows). The direct numerical solution of the kinetic equation is usually intractable due to the large number of independent variables. A useful alternative is to reformulate the problem in terms of the moments of the velocity distribution function. Closure of the moment equations is challenging for flows sufficiently far away from the Maxwellian limit. In previous work, a quadrature-based third-order moment closure was derived for approximating solutions to the kinetic equation for arbitrary Knudsen number. A key component of quadrature-based closures is the moment-inversion algorithm used to find the non-negative weights and velocity abscissas. Here, a robust inversion procedure is proposed for three-component velocity moments up to ninth order. By reconstructing the velocity distribution function, the spatial fluxes in the moment equations are treated using a kinetic-based finite-volume solver. Because the quadrature-based moment method employs the moment transport equations directly instead of a discretized form of the kinetic equation, the mass, momentum and energy are conserved for arbitrary Knudsen and Mach numbers. The computational algorithm is tested for the Riemann shock problem and, for increasing Knudsen numbers (i.e. larger deviations from the Maxwellian limit), the accuracy of the moment closure is shown to be determined by the discrete representation of the spatial fluxes.     

Vibration of L-shaped plates under a deterministic force or moment excitation: a case of statistical energy analysis application

Analytical and closed form solutions are presented in this paper for the vibration response of an L-shaped plate under a point force or a moment excitation. Inter-relationships between wave components of the source and the receiving plates are clearly defined. Explicit expressions are given for the quadratic quantities such as input power, energy flow and kinetic energy distributions of the L-shaped plate. Applications of statistical energy analysis (SEA) formulation in the prediction of the vibration response of finite coupled plate structures under a single deterministic forcing are examined and quantified. It is found that the SEA method can be employed to predict the frequency averaged vibration response and energy flow of coupled plate structures under a deterministic force or moment excitation when the structural system satisfies the following conditions: (1) the coupling loss factors of the coupled subsystems are known; (2) the source location is more than a quarter of the plate bending wavelength away from the source plate edges in the point force excitation case, or is more than a quarter wavelength away from the pair of source plate edges perpendicular to the moment axis in the moment excitation case due to the directional characteristic of moment excitations. SEA overestimates the response of the L-shaped plate when the source location is less than a quarter bending wavelength away from the respective plate edges owing to wave coherence effect at the plate boundary.     

Roton dipole moment

The roton excitation in the superfluid ⁴He does not possess a stationary dipole moment. However, a roton has an instantaneous dipole moment, such that at any given moment one can find it in the state either with positive or with negative dipole moment projection on its momentum direction. The instantaneous value of electric dipole moment of roton excitation is evaluated. The result is in reasonable agreement with recent experimental observation of the splitting of microwave resonance absorption line at roton frequency under external electric field.     

Roton dipole moment

JETP Letters - The roton excitation in the superfluid 4He does not possess a stationary dipole moment. However, a roton has an instantaneous dipole moment, such that at any given moment one can...     

Effect of baffle on gravity-gradient-excited slosh waves and spacecraft moment and angular-momentum fluctuations in microgravity

Summary The dynamical behavior of fluids affected by the asymmetric gravity gradient acceleration has been investigated. In particular, the effects of surface tension on partially filled rotating fluids applicable to a full-scale Gravity Probe-B Spacecraft dewar tank with and without baffles are studied. Results of slosh wave excitation along the liquid-vapor interface induced by gravity gradient acceleration indicate that the gravity gradient acceleration is equivalent to the combined effect of a twisting force and a torsional moment acting on the spacecraft. The results are clearly seen from one-up one-down and one-down one-up oscillations in the cross-section profiles of two bubbles in the vertical (r, z)-plane of the rotating dewar, and from the eccentric contour of the bubble rotating around the axis of the dewar in a horizontal (r, θ)-plane. As the viscous force, between liquid and solid interface, greatly contributes to the damping of slosh wave excitation, a rotating dewar with baffles provides more areas of liquid-solid interface than that of a rotating dewar without baffles. Results show that the damping effect provided by the baffles reduces the amplitude of slosh wave excitation and lowers the degree of asymmetry in liquid-vapor distribution. Fluctuations of angular momentum and fluid moment caused by the slosh wave excited by gravity gradient acceleration with and without baffle boards are also investigated. It is also shown that the damping effect provided by the baffles greatly reduces the amplitudes of angular momentum and fluid moment fluctuations.     

Are the gapless excitations responsible for the back bending of the moment of inertia?

An expression for the rotational frequency at which the gapless excitation arises in the nucleus has been found. The consequences of the gapless excitation resulting in the back bending of the moment of inertia have been analyzed.     

A new way of measuring the contribution of scattered photons to x‐ray fluorescence yields in secondary excitation mode

The characteristic K x‐rays from a secondary exciter in conjunction with an x‐ray tube or a radioisotope as primary source are frequently used for target excitation in x‐ray fluorescence measurements. In this experimental arrangement, the exciting photon spectrum consists of two parts: (i) characteristic K x‐rays of the secondary exciter and (ii) photons from the primary source, which are scattered from the secondary exciter. A new method is proposed for correcting the observed target x‐ray yield due to unwanted excitation by the scattered photons. This method involves measurement of x‐ray yield after replacing the secondary exciter by an equivalent exciter consisting of a low‐Z element foil with a high‐Z element backing. Copyright © 2004 John Wiley & Sons, Ltd.     

Generation of waves in an elastic plate by a torsional moment and a horizontal force

An approximate solution is determined for the motion of an infinite elastic plate, excited by a torsional moment (with the axis of the moment normal to the plate) and by a horizontal force (parallel to the plate). The driving moment and force are sinusoidal in time and applied to a small rigid indenter with a circular base, fixed to the plate. The solution is obtained from a three-dimensional approach but is evaluated only for low frequencies, where the wavelengths of the quasi-longitudinal, tranverse and bending waves are much larger than the thickness of the plate. For the case of excitation with a torsional moment, the solution contains two parts, one describing a travelling transverse wave and the other a local reaction. The local reaction is built up of two infinite sums of Love waves with imaginary wave numbers. The driving-point admittance due to the local reaction is larger than the admittance due to the tranverse wave when the diameter of the indenter is smaller than about twice the thickness of the plate. For the case of excitation with a horizontal force, the solution contains three parts that describe travelling waves (quasi-longitudinal, transverse and bending) and two parts that describe the local reaction (infinite sums of Lamb and Love waves). The admittances due to the three types of travelling waves are all of the same order of magnitude. The admittance due to the local reaction is of importance when the frequency is relatively high and the diameter of the identer much smaller than the thickness of the plate. For both cases of excitation, the admittance due to the local reaction increases with increasing thickness of the plate and tends to the value found for a semi-infinite medium.     

Investigation of area mobility of a finite thin plate subject to a uniform velocity excitation

Methods for theoretically calculating area mobility of a finite thin plate are derived for a typical excitation pattern,uniform velocity excitation,over a contact area.In the derivation,the contact area is discretised into sub-regions and the excitation and response continuously distributed over these sub-regions are simplified as acting at the centers of the sub-regions.Therefore,according to the concept of effective point mobility,the theoretical formula for area mobility of a finite thin plate subject to a uniform velocity excitation is obtained.According to the relation between complex power and area mobility,the measurement method of a finite plate is also obtained by means of transfer point mobility.When comparing results generated by theoretical calculation to patterns measured in experiment,consistent patterns are found,suggesting that the proposed methods are practical.The results of calculation show that the accuracy of theoretical calculation increases with the number of sub-regions and is influenced by the moment caused by the experiment device,especially when the excitation frequency is high.     

Moment Lyapunov exponents of a two-dimensional system under both harmonic and white noise parametric excitations

The moment Lyapunov exponents and the Lyapunov exponents of a 2D system under both harmonic and white noise excitations are studied. The moment Lyapunov exponents and the Lyapunov exponents are important characteristics determining the moment and almost-sure stability of a stochastic dynamical system. The eigenvalue problem governing the moment Lyapunov exponent is established. A singular perturbation method is applied to solve the eigenvalue problem to obtain second-order, weak noise expansions of the moment Lyapunov exponents. The influence of the white noise excitation on the parametric resonance due to the harmonic excitation is investigated.     

The design and dynamic performance of a moment actuator

I.IntroductionMachinenoiseisanenvironmcnta1po11utantanditscontrolremainsamajorunresolvedprob1cm.Inadditiontoair-bornesound,machinesimpartvibrationalenergyintosupportingandconnectedstructuresbybothtrans1ationa1androtationa1motions.Translationa1compo-nentsarerelative1yeasytoestimatesinccnearpureforcescanbegeneratedandmeasured.ThisisnotthecaseforrotationalcomponentSsincemomcntsaremoredifficu1ttogenerateandmeasure.Suchmeasurementswould1eadtoafu11undcrstandingofthis1ittleunderstoodbutoftenimporta…     

受均布速度作用的有限板面导纳研究

针对接触面积上典型的激励形式,均布速度激励,推导有限薄板上面导纳的理论计算方法。在推导过程中,将接触面积离散为较小的子面积,连续分布的激励和响应简化为作用在子面积中心的激励和响应,再利用有效点导纳的定义,获得有限薄板受均布速度激励的面导纳的理论计算公式;根据接触面积上复功率与面导纳的关系,推导出由传递点导纳获得有限板面导纳的测量方法。通过比较理论计算结果和实验测量结果,证明两种方法可行。对计算结果进行的分析还表明,子面积数的增加可以提高理论计算的精度,而测量精度在激励频率较高时受测量装置产生的弯矩影响较大。      

Absolute spin magnetic moment of FeF2 from high energy photon diffraction

The magnetic spin structure factor of FeF2 has been directly determined from high energy magnetic x-ray diffraction at 115 keV photon energy. A pure spin moment of mu = 4.01(5)mu(B) was observed, which agrees very well with the spin moment of the free Fe2+ ion and differs significantly from values of the total magnetic moment obtained by other methods. The magnetic phase transition of FeF2 has been carefully investigated and values for the critical exponent of the order parameter obtained by other techniques have been confirmed.     

Photo-induced persistent internal quadrupole moment of a semiconductor

The persistent internal quadrupole moment which has been observed in Li-doped ZnO crystals is studied theoretically. The quadrupole moment is expressed in terms of relaxation coefficients and effective cross sections characterizing the scattering and capture processes of the photo carriers. Furthermore, it is related to the anisotropy of the mobility and of the persistent internal polarization.     

采用系统函数分析换能器阻抗测量方法的性能

在分析传统三电压法测量阻抗优缺点的基础上,提出了一种发射换能器阻抗测量方法,以解决海上单模激发现场检修发射相控阵一致性的难题。该方法仍利用三电压法电路结构,但修改了测量参数。首先介绍了基于系统函数的换能器阻抗测量方法的原理,然后报道了当串联电阻选取不同数值时换能器阻抗测量的结果以及它们与高精度阻抗分析仪测量结果的比较,最后进行误差分析和测量曲线的等效电路参数拟合。通过对它的性能分析可以看到,在保证采样精度条件下,选择阻值相对换能器阻抗偏小的电阻值,其测量精度完全能达到高精度阻抗分析仪的精度。最后,通过对实验测量阻抗曲线的等效电路参数的最小二乘拟合表明,其拟合电参数准确,能应用到换能器匹配网络的设计。